The vessel was seized in the Indian ocean off East Africa on Sunday in the boldest attack by pirates operating from lawless Somalia.

"We can confirm the ship is anchoring off the Somali coast at Haradheere," Lieutenant Nathan Christensen, a spokesman for the US Fifth Fleet, said on Tuesday.

Haradheere is situated roughly in the centre of Somalia's coastline.

The supertanker had been heading for the US via the Cape of Good Hope at the southern tip of Africa, instead of heading through the Gulf of Aden and the Suez Canal.

The hijacking occurred despite an international naval response, including from the Nato alliance and European Union, to protect one of the world's busiest shipping areas.

US, French and Russian warships are also off the Somali coast.

The pirates have driven up insurance costs, forced some ships to go round South Africa instead of through the Suez Canal and secured millions of dollars in ransoms.

Last week, the European Union, in its first-ever naval mission, launched a security operation off the coast of Somalia to combat growing piracy and protect ships carrying aid agency deliveries.

Outrageous act

Prince Saud al-Faisal, Saudi Arabia's foreign minister called the hijacking of the Sirius Star an outrageous act and promised to back an EU-led initiative to step up security in shipping lanes off Africa's east coast.

The number of pirate attacks has doubledto 90 ships this year [EPA]

"This outrageous act by the pirates, I think, will only reinforce the resolve of the countries of the Red Sea and internationally to fight piracy," he told reporters in Athens.

The vessel owned by Saudi oil giant Aramco was fully loaded when it was attacked on Sunday more than 450 nautical miles southeast of Mombasa.

The standoff comes as another ship is seized off the coast of Somalia.

According to the International Maritime Bureau (IMB), a Thai fishing boat with 16 crew members has been hijacked.

Noel Choong, head of the IMB piracy reporting centre, based in Kuala Lumpur, said the ship was seized in the Gulf of Aden on Monday.

Eight ships have now been hijacked in the past two weeks.

'Hitting the jackpot'

Andrew Mwangura, co-ordinator of the East African Seafarers' Association, said: "The world has never seen anything like this ... The Somali pirates have hit the jackpot."

The association, based in the Kenyan port city of Mombasa, has been monitoring piracy for years.

Mwangura said he thought a hijacked Nigerian tug was a "mother-ship" for the November 15 seizure.

"The supertanker was fully loaded, so it was probably low in the water and not that difficult to board," he said, adding that the pirates probably used a ladder or hooked a rope to the side.

Pirates are well organised in the Horn of Africa, where Somalia's northeastern tip juts into the Indian Ocean.

Somalia has had no effective government since the 1991 overthrow of Mohamed Siad Barre, the former president, touched off a bloody power struggle that has defied numerous attempts to restore stability.

This year, Somali pirates have attacked 90 ships, more than double the number in 2007, according to the International Maritime Bureau, and are still holding 16 ships and more than 250 sailors.